1. Field of the Invention
The present invention relates to an on-vehicle radar device and on-vehicle radar control system, and in particular to an on-vehicle radar device and on-vehicle radar control system which avoid interference between radar devices by shifting the frequency band or transmission period of the monitoring signal.
2. Description of the Related Art
As vehicle preventative safety systems, a variety of on-vehicle radar devices have been proposed that detect the distance to a vehicle running ahead and the relative speed. As these radar devices become more widely adopted, there is a growing danger that as vehicles each of which has an on-vehicle radar device approach one another, interference between their radar devices will occur. To deal with this, a method is needed whereby interference between these on-vehicle radar devices can be avoided.
For example, in Patent Reference 1, a method is disclosed whereby in an FM-CW radar device that modulates the frequency of a monitoring signal, a transmitted signal and a received signal are mixed in a mixer, and interference is detected by measuring the frequency of the output signal of that mixer. Further, a method is disclosed whereby, upon detecting interference, the frequency band of the transmitted signal and/or the modulation cycle of the frequency is/are changed in order to avoid interference.
[Patent Reference 1] Japanese Patent Laid-Open Publication No. 2002-168947
However, with the conventional technique there is the following kind of problem. That is, even if in order to avoid interference among plurality of radar devices of the same model, the frequency band and/or the modulation cycle of the frequency are changed, if the operation of each is not done in an organized manner, eventually interference will reoccur. In other words, if after changing, the frequency bands of the transmitted signals and/or the modulation cycles are not different one to the other, it is not possible to avoid interference with certainty.
Also, although it is true that interference could certainly be avoided if each radar device were allocated its own frequency band, it is not realistic to allocate in the commercial-use frequency band of 76.0 GHz to 77.0 GHz a different band to all the countless radar devices that exist.